This invention relates to metallurgy and more particularly to the diffusion bonding of metals.
Surfaces of all aluminum alloys are characterized by a very thin, adherent, and continuous coating of aluminum oxide (Al.sub.2 O.sub.3). In fact, it is this coating or film which gives Al and its alloys the excellent properties such as oxidation resistance, corrosion resistance and stability which most steels do not possess. Another unusual characteristic of Al alloys is that the film forms immediately, even in hard vacuum. In air, the film also forms immediately, grows to 100-500 angstroms and remains stable. It is for this reason that welding and soldering of Al alloys is considerably more difficult than steels and copper and nickel base alloys. Soldering of Al alloys can be accomplished only when the oxide is directly displaced during the process. In welding, the oxide film must disturbed by brushing just prior to passing the inert gas enveloped electric arc to form a good joint. Even when Al is in molten condition, the film will stay on the top of the surface and prevent runaway oxidation.
Thus, the presence of the surface oxide on Al, while very advantageous for many purposes, is a significant barrier to joining, particularly in the solid state. In contrast, titanium alloys, notably, superplastic Ti-6Al-4V can be joined in the solid state at approximately 900 .degree. C under pressures of 600-1000 psi. Very complex shapes can be formed by this method without the necessity of welding or riveting Many Al alloys are also superplastically formable, but are not bondable in the solid state. The primary attribute of Ti alloys is that the oxide of titanium (TiO.sub.2) is soluble in metallic Ti. Therefore, when two sheets of superplastically deformable Ti-6Al-4V are processed, they are metallurgically bonded together nearly perfectly. The oxide of Ti on the surface is actually dissolved in the alloy to create a fresh metallic Ti leading to a sound bond between the two mating surfaces On the other hand, Al does not behave in the same manner because its oxide tenaciously remains on the surface. Superplastic forming and diffusion bonding (called SPFDB) is possible in the case of Ti alloys while Al alloys need surface modification to remove or disturb the Al.sub.2 O.sub.3 film. Numerous efforts to achieve SPFDB of Al alloys can be cited but success has been elusive.
There is an urgent need to develop processes which enable diffusion bonding of superplastically formable Al alloys because the need for rivets would be reduced or eliminated. This would result in the improvement of performance and safety of military and commercial aircraft. Performance would be improved by the reduction in weight of structures; safety would be the result of increased fatigue life. This has been demonstrated by the use of superplastic forming and diffusion bonding of titanium structures and parts in military aircraft, including the F14 and F16. Application of superplastic forming and diffusion bonding to aluminum alloys is bound to be even more useful than that of titanium alloys because approximately 30 to 50 percent of the modern military and commercial aircraft are composed of aluminum based materials.
For other applications it would be desirable to provide a procedure for the simple diffusion bonding of aluminum or aluminum alloy pieces together It would also be desirable to provide a method for diffusion bonding graphite/aluminum metal matrix composite pieces together.